1. FIELD OF THE INVENTION
The present invention relates to an automatic transmission to be mounted on an automobile especially to an automatic transmission having a forward four speed automatic transmission mechanism. In detail, it relates to a hydraulic device used when a hydraulic actuator to operate a clutch is drained.
2. DESCRIPTION OF THE RELATED ART
The applicant of the present invention has proposed an automatic transmission having an automatic transmission mechanism of forward three or four speeds composed of a planetary gear unit made up of the combination of a single planetary gear and a dual planetary gear. The invention by the above applicant is thought out to use the common parts and the assembly process between the mechanism of forward three speeds and that of four speeds in order not to raise so much production cost. And this automatic transmission is thought out to answer for the wide variation for the vehicle types.
The applicant of the present invention has proposed an automatic transmission mechanism, wherein a fourth clutch (C3) is placed to work with a one-way clutch which operates at the third and the fourth speed modes to prevent mechanical locking to the reverse direction. (refer to the Japanese application, No. Sho-62-094868).
In the automatic transmission mechanism 1', as shown in FIG. 5, a one-way clutch OF operating in the transmission from the fourth speed mode to the third speed mode, clutches C1 and C2 connected to rotating elements R1 and R2 respectively, a third clutch CO, a fourth clutch C3, hydraulic actuators 3, 12 and 54 for controlling above clutches are all housed in a clutch section 47 which is placed in a rear portion of the mechanism 1' Further, in the clutch section 47, a first (forward) clutch C1 is placed between a spline arranged on the inside surface of the peripheral section of a clutch drum 4 and a spline arranged on the axially elongated section of a ring gear R1. A second (reverse) clutch C2 is placed between a spline arranged on the inside surface of a peripheral section of a movable member 7 and a spline arranged on the outer surface of a hub 50a fixed on the hollow shaft. The movable member 7 is enclosed by a clutch drum 4 through a spline to permit an axial movement and the movable member 7 encloses a piston member 15. The movable member 7 and a cylinder made up by the inside surface of the clutch drum 4 constitute an oil chamber, which is a hydraulic actuator 3 for the first clutch C1. The piston member 15 and a cylinder made up by the inside surface of the movable member 7 constitute an oil chamber, which is a hydraulic actuator 12 for the second clutch C2. Between the piston member 15 and the receiving member 16 fixed on a sleeve 5a by a snap ring, a spring 10 is arranged. The spring 10 composes a return spring used for both the piston members 7 and 15 of hydraulic actuators 3 and 12.
When engaging and operating the first (forward) clutch C1, the movable member 7 is moved inside the clutch drum 4 by the hydraulic pressure taken out from the oil hole 13 arranged in the sleeve 5a. When engaging and operating the second clutch C2, the piston member 15 is moved by the hydraulic pressure taken out from the oil hole 14 arranged in the sleeve 5a. And when releasing the first clutch C1, the piston member 15 and the movable member 7 are moved to the side of the clutch drum 4 by the return spring 10 and are operated to contract by draining the hydraulic actuator 3.
In the automatic transmission mechanism 1' which has been proposed by the applicant of the present invention the first (forward) clutch C1 is engaged up to the third speed mode and the clutch C1 is released when shifting up to the fourth speed mode. The clutch C1 must be released quickly by draining the hydraulic actuator 3 speedily at the high speed rotation to shift to the fourth speed mode. At this state, the actuator 3 rotates at a high speed with the input shaft and the centrifugal hydraulic pressure is working strongly, so it is difficult to drain the oil smoothly. In order to drain the centrifugal hydraulic pressure, it is possible to arrange many check balls in the movable member 7, however, by this structure slip time is made longer because certain time is needed for increase of the hydraulic pressure when the hydraulic pressure is supplied at a high speed rotation. Furthermore, if the hydraulic pressure raise cannot overcome the centrifugal force working on many balls, the connection of the clutch C1 is not done easily because of the difficulty of the hydraulic pressure raise.